On a worldwide basis, hepatocellular carcinoma (HCC) is the tenth most deadly cancer-related killer. Even with the advancement and combination of surgery, radiation and chemotherapy, the prognosis for HCC remains poor (1). The 5-year survival rate for individuals with liver cancer in the United States is only 8.9% despite aggressive conventional therapy, marking this malignancy the second most lethal cancer after pancreatic ductal adenocarcinoma (4.4% survival at 5 years) (2). In 2005, there were over 667,000 new cases of liver cancer worldwide with 80% in Asia and sub-Saharan Africa (3). With tremendous progress in the field of molecular oncology, novel treatment strategies are constantly being developed in attempts to cure this disease.
The development of targeted therapeutics against cancer, with improved discrimination between tumor cells and nonmalignant counterparts, is one of the major goals of current anticancer research. Most chemotherapeutic agents do not preferentially accumulate at the tumor sites. Indeed, the dose that reaches the tumor may be as little as 5%-10% of the dose accumulating in normal organs (4). The toxic side effects often limit dose escalation of anticancer drugs, leading to incomplete tumor response, early disease relapse, and ultimately, the development of drug resistance. Several approaches were developed to improve the selective toxicity of anticancer drugs such as encapsulating anticancer drugs in delivery systems (5) and targeting anticancer drugs via monoclonal antibodies (6, 7) or peptide ligands (8, 9) that bind to antigens or receptors that are over-expressed, or uniquely expressed on the cancer cells.
Drug delivery systems (DDS) such as lipid or polymer based anti-cancer nano-medicines have been investigated (10). DDS usually refers to nanoparticles and microparticles with diameters of 200 nm or less including liposomes and other lipid based carriers such as micelles, lipid emulsions, and lipid-drug complexes; also included are polymer-drug conjugates and various ligand targeted products such as immunoconjugates (11). The hyper-permeability of tumor vasculature is one of the key factors governing the successful targeting of a tumor by polymer-based cancer therapies (12). After intravenous administration, the ‘leakiness’ of the angiogenic tumor vasculature, estimated to have an average pore size of 100-600 nm (13), allows selective extravasation of the conjugate in the tumor tissue. Additionally, tumor tissue frequently lacks effective lymphatic drainage, which subsequently promotes polymer retention. The combination of these factors leads to an accumulation of the conjugate in tumor tissue—a passive targeting phenomenon named by Maeda as the ‘enhanced permeability and retention (EPR) effect’ (14). EPR-mediated passive tumor targeting by liposomes can result in several-fold increases of drug concentration in solid tumors relative to those obtained with free drugs (15).
The particular strength of DDS is their potential to alter the pharmacokinetics and the biodistribution of their associated therapeutics (5). Coupling of polyethylene glycol (PEG) or other inert polymers to a variety of therapeutic molecules may decrease drug clearance by the kidneys and by the reticular endothelial system (RES) (16). For larger particulate carriers, such as liposomes and polymer-drug conjugates, the size of the carrier (generally 50 to 200 nm in diameter) confines it mainly to the blood compartment, with less pernicious effects on normal organs.
The majority of the DDS currently approved for parenteral administrations include liposomal or lipid based formulations and therapeutic molecules linked to PEG, for instance, PEGylated liposomal doxorubicin, which was used to treat highly angiogenic tumors such as AIDS-related Kaposi's sarcoma, with overall response rates of 43% and 59% (17, 18). However, particulate DDS cause increased accumulation of drugs in mononuclear phagocytic system cells in the liver, spleen, and bone marrow, and the possibility exists for increased toxicities to these tissues (19). Moreover, with the increased circulation time and confinement of the particulate DDS, hematological toxicities such as neutropenia, thrombocytopenia, and leucopenia have also become apparent (20). Efforts are being made to enhance the site-specific actions of DDS by combining them with ligands targeted to tumor cells and tumor vasculature surface antigens or receptors, a process called active- or ligand-mediated targeting (8, 21). In addition, the delivery of chemotherapeutic drugs to tumor tissue through affinity targeting is being investigated (22, 23).
Although monoclonal antibodies have shown clinical potential as tumor targeting agents, poor tumor penetration of the antibodies due to their size, and liver/bone marrow toxicity caused by non-specific antibody uptake are the two major limitations of antibody therapy. Peptide-targeting agents may ease the problems associated with antibody cancer therapy (24). Combinatorial libraries displayed on microorganisms are a possible strategy to identify tumor specific targeting ligands.
Phage display technology has been applied to identify B-cell epitopes (25-27), discover tumor cell (8, 28, 29) and tumor vasculature specific peptides (30-33). Combining DDS with tumor specific peptides may lead to up to several thousand anticancer drug molecules delivered to tumor cells via only a few targeting ligand molecules. The sustained release of the anticancer drug molecules at the tumor site may also have therapeutic advantages (8, 34).